Fuel cell power plants include a stack of fuel cells, each cell having anode and cathode catalysts surrounding an electrolyte, such as phosphoric acid or a proton exchange, polymer electrolyte membrane. In either case, one product of the process is water, which must be removed from the cathode in order to prevent flooding. In PEM fuel cells, it is necessary to keep the anode side of the membrane from becoming dry, a condition which impedes the process of generating electricity. Therefore, water management is important.
In many applications, including stationary applications, space applications and vehicular applications, fuel cell power plants may be utilized in freezing environments, that is, where temperatures will fall below 0° C. (32° F.). When the fuel cell power plant is operating, it produces sufficient heat to maintain all of the water in the system above freezing; however, when the fuel cell power plant is inoperative at subfreezing temperatures for sufficient periods of time, water may freeze resulting in the potential for physical damage as well as causing the fuel cell power plant to be initially inoperable when startup thereof is desired.
In vehicular applications, a desirable criteria is the ability to operate the vehicle, with electricity generated by the fuel cell power plant, in less than one minute (or even less than 40 seconds) following the initial indication of startup, such as by turning of a key or other switch.
In the prior art, procedures have been devised for draining all or part of the water from water flow systems into an accumulator at shutdown, thereby to avoid mechanical damage as a result of ice formation in some or all of the components of the fuel cell stack and its related water management system. However, upon startup, especially at temperatures below −20° C. (−70° F.), melting of the ice in the accumulator and in the associated pumps, conduits and heat exchangers (if not drained) is necessary in order to provide water management and/or coolant to the fuel cell power plant before operation thereof can be sustained.
In U.S. Pat. No. 6,797,421, the fuel cell stack, the accumulator, the water pump and heat exchanger are all disposed within an insulated enclosure which is heated by a hydrogen/oxygen catalytic combustor, which is either disposed within the enclosure, or externally thereof, the heat being communicated via a small inlet duct or hood in the lower portion of the enclosure. In this known apparatus, one or more temperature sensors are disposed at various critical points within the enclosure, the temperature indicating signal(s) of which are provided to a controller which in turn controls either static or intermittent flow of hydrogen through a valve to the catalytic combustor. There is no disclosure of controlling the amount of air that is provided to the combustor. The controller, of course, must be provided with electrical power throughout the time that the fuel cell power plant is inoperative at subfreezing temperatures, the source of such power not being disclosed. The temperature of the catalytic combustor therein is set to operate between 93° C. (200° F.) and 370° C. (700° F.), which causes inefficient operation of the combustor.
In U.S. 2002/0068202, it is suggested that a fuel cell power plant (referred to therein as a battery) can be operated with a minimal load, either continuously or in response to a temperature sensor, so that the electrolyte of the fuel cells remain above freezing.
There are situations other than within the framework of fuel cell power plants wherein maintaining a suitable temperature of a medium in a container is required.